The invention relates to a holder for a glow starter switch in electric discharge lamps. More specifically, the invention relates to a metallic annular holder for the glow starter which prevents overheating of the glow starter when the holder is subjected to an electromagnetic field.
Electric discharge lamps are often provided with a glow starter switch, which when connected to a ballast, provides a high voltage pulse across the electrodes which aid in starting the discharge device. The glow starter is normally supported within the outer envelope of a discharge lamp by a metallic holder which encircles the bulb of the glow starter and which is welded to one of the frame members or current-supply conductors supporting the discharge device. One type of glow starter comprises a glass bulb filled with a predetermined gas, a pair of fixed contact electrodes, and a bimetallic contact. During starting, the voltage applied at the glow starter produces a glow discharge between the contacts. The glow discharge heats the bimetallic strip which flexes and bridges the contact electrodes. This short-circuits the current-supply through the ballast and also extinguishes the glow discharge in the glow starter switch. The bimetallic contact then cools and breaks contact with the contact electrodes which interrupts the short circuit current through the ballast and causes a high-voltage spike across the electrodes.
Electric discharge lamps are provided with getters for removing trace amounts of undesired gases remaining within the outer lamp envelope after it has been sealed and evacuated to a predetermined pressure. The getter often is disposed on a disc-shaped support which is secured to one of the current-supply conductors extending from the lamp stem by a wire leg extending from the getter support. The disc-shaped getter support is positioned at a predetermined orientation with respect to the lamp axis and defines a getter plane. When two getters are included in the lamp, the disc-shaped supports for each are often positioned in a common getter plane. The term "getter" will hereinafter refer to the disc-shaped getter support with the getter material disposed thereon.
To eliminate the trace impurities remaining within the evacuated outer envelope, the getter material is activated by heating the getter in a radio frequency (R.F.) electromagnetic field for several seconds, which is known as "flashing" of the getter. The sealed outer envelope is positioned near an electromagnetic source such that the field lines of the electromagnetic field are transverse to the disc-shaped getter supports to achieve maximum coupling between the getter support and the applied field, and therefore maximum heating of the getter material.
A common arrangement is to secure the getter supports in the lamp transverse t the lamp axis. The sealed lamp is then positioned within a circular coil which is energizable for producing an R.F. electromagnetic field. The lamp is positioned with the getter supports parallel to and in the same plane as that of the coil. In this position, the getters are in the region of the greatest field strength and are also transverse to the magnetic field.
However, the electromagnetic field also interacts with the metallic glow starter holder and induces an alternating current in the holder. The induced current greatly increases the temperature of the holder, often to a point such that portions of the holder glow. These portions are the parts of the holder transverse to the electromagnetic field and closest to the getters.
Heating of the glow starter holder is undesirable because the hottest parts of the holder can damage the glow starter by cracking the glow starter bulb. Alternatively, the heat from the holder can set free impurities in the glass of the glow starter bulb, which then contaminate the gas inside the glow starter bulb and deteriorate the performance of the glow starter.
The getter and the glow starter are normally positioned within the outer lamp envelope near the lamp base so that light from the discharge device is not obstructed. Another reason for the position of the getters near the lamp base is to ensure that the getter film resulting from the flashing of the getters is deposited on the inner surface of the lamp envelope near the lamp base so that the film does not obscure light from the discharge device. The proximity of the getter and glow starter is thus determined by the size of the outer envelope in the region of the lamp cap. Accordingly, it is usually not a viable alternative to space the glow starter and metallic holder a sufficient distance from the getters such that it is not overheated by the electromagnetic field during flashing of the getters.